In recent years, the use of organic electroluminescent (EL) elements has been investigated in display devices such as a flat display and light sources in electrophotographic copiers and printers.
The above organic EL elements are electric current driving type light emitting elements in which a very thin film of fluorescent organic compounds is sandwiched between an anode and a cathode, and light is emitted due to flow of the electric current. Generally, organic materials are insulators, but it is possible to inject an electric current when the thickness of the organic layer is markedly decreased, whereby it is possible to drive it as an organic EL element. Further, at a low voltage of at most 10 V, it is possibly to achieve driving. Due to that, it is possible to realize highly efficient light emission, whereby attention has been paid as a future display.
Specifically, S. R. Forrest et al. have recently discovered a phosphorescence emitting organic EL element utilizing an excited triplet state, realizing an efficiency, which significantly exceeds that of conventional organic EL element utilizing a singlet state (Appl. Phys. Lett. (1999), 75(1), 4-6). Further, as reported by Adachi et al. (J. Appl. Phys., 90, 5048 (2001)), luminous efficiency reached even 60 lm/W and the resulting element is expected to be applicable not only to display but also for applications as lighting sources.
When lighting devices employing an organic EL elements are produced, it is essential to consider the following aspects.
At present, organic EL materials are of low molecular weight based types and also polymer based types. Production of organic EL elements employing low molecular weight materials are carried out via evaporation under high vacuum. Low molecular weight materials are easily purified due to capability of sublimation purification, whereby it is possible to employ highly pure organic EL materials. Further, since it is easy to realize a laminated layer structure, whereby excellent efficiency and shelf life are realized.
However, since vapor deposition is carried out under high vacuum of at most 10−4 Pa, operation becomes complex, resulting in an increase in cost, whereby preference is not always realized in terms of production. Specifically, for lighting applications, it is essential to form an organic EL element of a large area, whereby vapor deposition becomes a difficult production process. Further, in regard to a phosphorescent dopant which is employed in phosphorescence emitting organic EL elements, it is difficult to introduce, to an organic EL element, a plurality of uniform dopants of a large area via vapor deposition. Consequently, in terms of cost and technology, such process is currently considered to be very difficult.
Contrary to the above, in regard to polymer based materials, for their production, it is possible to employ wet processes such as spin coating or ink-jet printing. Namely, since it is possible to carry out production under atmospheric pressure, advantages result in which cost is lowered. Further, since a thin film is produced employing a previously prepared solution, features result in which dopants are easily regulated and non-uniformity tend to not result even in a large area. It may be stated that the above features are very advantageous in the aspect of cost and production technologies for lighting use of organic EL elements.
However, when the wet process is employed in the polymer based materials, it is difficult to realize a laminated layer structure. When a second layer is laminated onto a first layer, the polymer materials are dissolved in solvents of the second layer to result in blending of the first and second layers. Due to that, compared to production of elements employing low molecular weight materials, production efficiency is commonly degraded.
Polymer based organic EL elements are commonly produced via a spin coating method, an ink-jet method or printing.
The spin coating method is only applicable to sheets, making it impossible to achieve continuous production. The ink-jet method is very useful for production of the display via a three-color light emitting system, but is not preferable, in terms of productivity, for production of light emitting devices such as displays for lighting or via a color exchange system, in which one side is subjected to light emission of the same color.
Consequently, production employing a printing method, has been proposed (refer, for example, to Patent Documents 1-3). The printing method is a useful film forming method due to its high simplicity and is a method of forming layers one by one. Consequently, if formation of a multi-layered structure is demanded, it is necessary that after making one layer, the following layer be formed. Due to that, productivity is low, and further, the apparatus is large and the number of processes increase, whereby the resulting cost increases. Further, when polymers, which are soluble in organic solvents, are laminated, a problem occurs in which two adjacent layers are blended.    Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 3-269995    Patent Document 2: JP-A No. 10-77467    Patent Document 3: JP-A No. 11-273859